1. Field of the Invention
The present invention relates to electronic devices, and in particular, to the testing of electronic devices.
2. Description of Related Art
Control of static charge in test and assembly of semiconductor components is an activity with increasing importance as the components become more sensitive to electrostatic discharge (ESD). Common in test and assembly is the practice of monitoring certain equipment that might produce damaging ESD events, such as socketing of a charged component. One known method for monitoring electrostatic discharge is to use a near-field ball antenna. The antenna is positioned approximately 15–30 cm from the equipment and components to be monitored. An oscilloscope is used to pick up electromagnetic interference (EMI) signal caused by an ESD event. Components tested under a charged device model (CDM) are typically subjected to simulated ESD pulses using an automated CDM testing process.
The EMI signals picked up by the prior art antenna have an oscillatory waveform. This is because the component under test is coupled to a metal ground. The electrostatic discharge from the ESD event is discharged into the metal ground and the oscillations generated occur in the metal ground. These oscillations occur despite the fact that the ESD event is a largely unipolar event (i.e., a brief current spike) from the point of view of the charged component generating the electrostatic discharge. This is because the component is a small electronic component relative to the much larger metal ground. The sudden injection of current into the large metal ground causes ringing in that metal ground, with the resulting radiation (EMI signals) being picked up by the near-field ball antenna. Having spread into the large expanse of the metal ground, the charge does not find its way back into the small component in a significant way, so the unipolar pulse at the component is consistent with continuing electrical oscillations in the metal ground as the energy dissipates.